1. Field of the Invention
The present invention relates to a frequency control circuit, a motor drive apparatus, a frequency control method, a control method of the motor drive apparatus, and a program allowing a computer to execute the control method, which allow a mobile member brought into contact with a vibrator vibrated by a piezoelectric element to relatively move by a friction force.
2. Description of the Related Art
Heretofore, in general, an oscillatory wave (vibration type) motor called as an ultrasonic motor or a piezoelectric motor has been developed and put to practical use. The oscillatory wave motor, as is well-known, is a motor configured to apply an alternating voltage to an electromechanical energy conversion element such as the piezoelectric element or an electrostrictive element so as to allow the element to generate high frequency vibration, thereby taking out vibration energy as a consecutive mechanical movement. The principle of operation of the oscillatory wave motor is already known in many documents such as Japanese Patent Application Laid-Open No. H03-289375 (corresponding to U.S. Pat. No. 5,656,881), and therefore, the description thereof will be omitted.
FIG. 12 is an external view of a stick ultrasonic motor according to a conventional example and the taking out of supply of voltage and output of voltage of the piezoelectric element.
In FIG. 12, an ultrasonic motor 100 includes a vibrator 101 having an insulating sheet 104 provided at an end surface thereof, a rotor 102, and an output gear 103. The vibrator 101 is configured by a combination of the piezoelectric element or the electrostrictive element and an elastic member. The vibrator 101 is configured to include the piezoelectric elements A1 and B1, electrode plates A-d, A′-d, B-d and B′-d sandwiching the elements A1 and B1, a vibration detection element S1, an electrode plate S-d, and an insulating sheet 105, and to be four-phase driven (blocks A, A′, B and B′). The drive signal of the block A is fed to the electrode plate A-d, the drive signal of the block A′ fed to the electrode plate A′-d, the drive signal of the block B fed to the electrode plate B-d, and the drive signal of the block B′ fed to the electrode plate B′-d. There exists no common electrode (GND).
In the above described configuration, the blocks A and A′ and the blocks B and B′ are fed with the DUTY 50% drive signals of reversed phases, to be driven so that the voltages with reverse phase are applied to both ends of the piezoelectric elements A1 and B1, respectively.
FIG. 13 is a block diagram showing the configuration of the drive circuit of the stick ultrasonic motor 100 of FIG. 12.
In FIG. 13, the drive circuit includes switching circuits 110a and 110b, a microcomputer 111, a voltage detection circuit 112, a phase difference detection circuit 113, inductance elements 114 and 116, and capacitance elements 115 and 117. The switching circuits 110a and 110b use FFTs as the switching elements. Each FFT is provided with a diode that allows a current flowing in a reverse direction to pass through. By providing this diode, the FFT is prevented from being damaged by the current flowing in the reverse direction.
The inductance elements 114 and 116 and the capacitance elements 115 and 117 are the elements that adapt impedance to the ultrasonic motor 100. By providing the impedance elements to the positions shown in FIG. 13, it is possible to drive the ultrasonic motor 100 at a low voltage and at high efficiency by the four-phase driving method described in FIG. 12. Note that the capacitance elements 115 and 117 are not necessarily included.
The voltage detection circuit (for example, A/D converter) 112 detects a magnitude of Vbat of the power supply voltage fed to the switching circuits 110a and 110b, and loads it into the microcomputer 111. In practice, based on a detection result of the voltage detection circuit 112, a pulse width and the like of the drive signal are changed, and an input power for the ultrasonic motor 100 is controlled.
FIG. 14 is an external oblique view of the vibrator of an oscillatory wave actuator proposed aiming at much smaller size.
In FIG. 14, the vibrator 201 of the oscillatory wave actuator includes an elastic member 204 made of a metallic material and formed in the shape of an oblong plate, a piezoelectric element (electro-mechanical energy conversion element) 205 joined to the rear surface of the elastic member 204, and projection portions 206 disposed on the top surface of the elastic member 204. This oscillatory wave actuator is disclosed in Japanese Patent Application Laid-Open No. 2004-320846 (corresponding to U.S. Pat. No. 7,109,639), and therefore, the detailed description thereof will be omitted.
The projection portion 206, as will be described below, moves a driven body by having the top ends thereof contacted to the driven body. The vibrator 201 can excite vibration of bending vibration modes, and by combining these two bending vibration modes, an elliptic motion can be generated at the top end of the projection portion 206.
FIG. 15A is a view showing one bending vibration mode, and FIG. 15B is a view showing the other bending vibration mode.
In FIGS. 15A and 15B, the vibration mode shown in FIG. 15A represents one bending vibration mode (referred to as mode A) of the two bending vibration modes. The mode A indicates a secondary bending vibration in a long side direction (direction of an arrow mark X) of the vibrator 201 (elastic member 204), and has three nodes parallel with a short side direction (direction of an arrow mark Y). The projection portion 206 is installed in the vicinity of a position where a node appears in the vibration of the mode A, and performs a reciprocating motion by the vibration of the mode A in the direction to the arrow mark X. By installing the projection portion 206 in this manner, the projection portion 206 can be displaced at the maximum in the direction to the arrow mark X.
The vibration mode shown in FIG. 15B represents the other bending vibration mode (referred to as mode B) of the two bending vibration modes. The mode B indicates a primary bending vibration in a short side direction (direction of an arrow mark Y) to the vibrator 201 (elastic member 204), and has two nodes parallel with the long side direction (direction of the arrow mark X). Here, the nodes in the mode A and the nodes in the mode B are approximately orthogonal to one another within X and Y planes. The projection portion 206 is installed in the vicinity of a position where a loop appears in the vibration of the mode B, and performs a reciprocating motion in the direction of an arrow mark Z by the vibration of the mode B. By installing the projection portion 206 in this manner, the projection portion 206 can be displaced at the maximum in the direction of the arrow mark Z.
That is, as described above, by allowing the nodes of the modes A and B to be approximately orthogonal to one another, the positions of the nodes of the mode A and the positions of the loop of the mode B can be matched. By installing the projections 206 at these positions, the vibration displacement of the projections 206 can be made to the largest extent, thereby obtaining high output. As described above, by displacing the projection portions 206 in X and Z directions on a large scale, the driven body contacting the projection portions 206 can be given a large driving force.
FIG. 16 is an external oblique view of the oscillatory wave actuator including the vibrator of FIG. 14.
In FIG. 16, the oscillatory wave actuator includes a vibrator having the elastic member 204, the piezoelectric element 205, and the projections 206, and a slider 207. The vibrations of the mode A and the mode B are generated with the predetermined phase differences therebetween, so that the elliptic motion can be generated at the top end of the projection portion 206. The top end of the projection portion 206 is pressure-contacted with the slider 207, which is the driven body. The slider 207 can move in a direction to an arrow mark L by the elliptic motion of the projection portion 206.
If the two projections 206 are symmetrically installed with respect to an XZ plane or a YZ plane that passes through the center of the elastic member 204, a reaction force received from the slider 207 in the projection portion 206 can be received by the vibrator without deviation. Because of stabilizing a relative position relationship between the slider 207 and the projection portion 206, the output of the vibrator can be stabilized without being affected by fluctuation and the like of environment and load.
The miniature oscillatory wave motor capable of exciting the two bending vibration modes in such a simple structure can separately generate two bending vibration modes. Hence, by controlling the magnitude of the voltage applied to the piezoelectric element 205 and the phase difference of two driving signals corresponding to each of the two bending vibration modes, the moving velocity of the slider 207 can be easily changed.
However, when the oscillatory wave motor is miniaturized and the driving frequency of the oscillatory wave motor becomes high, the driving frequency and a control step of the phase difference of the driving signals of the two bending vibrations become rough in the conventional oscillatory wave motor driving circuit that creates an oscillating frequency by dividing a digital clock. Hence, there arises a problem that, in the vicinity of a resonance frequency of the oscillatory wave motor, it is impossible to control the driven body (slider) by a minute moving velocity step.